The invention relates to semiconductor wafer storage apparatuses that protect semiconductor wafers from environmental pollution before and after each semiconductor fabricating process as well as to a semiconductor device fabricating apparatus to which such apparatuses are applied.
Amid progress in miniaturization and higher integration of semiconductor devices typically represented by memories, not only state-of-the-art processing techniques have been employed, but also the number of processing steps has been increased for their fabrication. Further, miniaturization demands a variety of measures to be taken to cope with various kinds of pollution affecting wafers under semiconductor fabrication processes.
Since pollution of the fabricating environment leads to a decrease in the yield of semiconductor devices, the semiconductor devices are produced within a clean room that is called by such designation as class 1, class 10, or the like defined in accordance with fabricating requirements.
While the fabrication of semiconductor devices involves a number of processes, it is required that wafers be temporarily stored in a clean room before and after each process. Therefore, wafers are stored in a wafer storage unit having an automated carrying function while contained in a number of wafer cassettes.
The size of the wafer storage unit is increased because of the need for a mass production of semiconductor devices and because of the fact that wafers are forwarded within the clean room through an overhead forward car or the like. That is, the wafer storage unit is 2 to 3 meters high in general.
An exemplary wafer storage unit, which is disclosed in the Examined Japanese Patent Publication No. Hei. 1-41561, will be described with reference to FIG. 8.
In FIG. 8, reference numeral 1 denotes a clean room that blows down clean air 4 from a ceiling 2 via a ULPA (ultra low penetration air-filter) filter 3 that is ordinarily used in clean rooms and sends the downwardly flowing air below the floor via a grated floor 5. Reference numeral 100 denotes a wafer storage unit; 6, a shelf; 7, a wafer cassette that contains a semiconductor wafer and that is accommodated in each compartment of the shelf 6; and 8, a wafer cassette 7 forward unit. In a shelf main body 9, filters 10 are arranged in the back of the shelf 6, and air supply units 11, each including a fan for supplying air 12 to the rear side of the wafer cassettes 7, are arranged below the main body 9. The wafer storage unit 100 is of the open type within the clean room while ventilated by the air supplied from the clean room.
An operation of the wafer storage unit will be described next.
During the operation of forwarding the wafer cassettes 7 to and from the shelf 6 by the forward unit 8, the clean air flows downwardly. The clean room not only sucks such downwardly flowing air from below the floor 5, but also supplies the clean air 12 past the filters 10 from the back of the shelf 6. As a result, entrance of dust from the forward unit 8 into the shelf 6 that contains the wafer cassettes 7 can be blocked.
As described above, the conventional wafer storage unit is designed to prevent semiconductor wafers from being contaminated by dust by means of the air that ventilates the clean room. However, the ULPA filter that is usually used in clean rooms cannot remove organic mists, moisture, and the like, and therefore the quality of semiconductor wafers has been adversely affected by the contamination derived from such substances.
Contamination has been brought about by the following substances.
(1) Organic Mists
Organic substances comprising elements such as phosphorus and boron separated and scattered around from the materials of which the clean room is constructed deposit on the semiconductor wafers within the wafer storage apparatus that is of the open type within the clean room. If such wafers are loaded into a diffusion furnace to be heated to high temperatures, the organic substances are also diffused into the wafers, thereby not allowing the desired semiconductor quality to be obtained.
(2) Moisture
Moisture within the clean room air that cannot be removed by the ULPA filter deposits on the wafers.
The depositing moisture forms naturally oxidized films on the semiconductor wafers. The thicknesses of such naturally oxidized films differ depending on the storage period, clean room environment, and the like.
Therefore, during the oxide film forming process which is one of the semiconductor fabricating processes, an oxide film having a desired thickness is formed on a naturally oxidized film having unspecified thickness, which makes the thicknesses of the oxide films nonuniform. As a result, higher performance and miniaturization needs for semiconductor devices cannot be met.
(3) Further, higher integration of semiconductors requires an excimer laser or the like be used as a light source in the lithographic process. If an organic substance is adsorbed on the resist of a stored wafer, a less-soluble layer is formed on the resist surface and the resist film is reduced, which deactivate the acid of the resist due to interaction between the organic substances and the laser beam. As a result, miniaturized patterning cannot be implemented.
The conventional wafer storage unit is of the open type within the clean room and is designed to protect semiconductor wafers from dust produced by the operation of the forward unit within the wafer storage apparatus by ventilating the clean room by the clean room air. However, organic mists that cannot be removed by the ULPA filter that is usually employed in clean rooms affect the quality of products by, e.g., depositing on semiconductor wafers and being diffused into the substrate in the following process, or by impairing patterning in the lithographic process. Further, moisture causes an oxide film of unspecified thickness to be formed to affect the quality of products. Still further, the semiconductor fabricating equipment entails large investment, which has imposed a great burden on the part of the manufacturer.